Optical article and process for forming article

ABSTRACT

The optical article of the invention, e.g., holographic recording medium or polymeric waveguide, is formed by mixing a matrix precursor and a photoactive monomer, and curing the mixture to form the matrix in situ. The reaction by which the matrix precursor is polymerized during the cure is independent from the reaction by which the photoactive monomer is polymerized during writing of data. In addition, the matrix polymer and the polymer resulting from polymerization of the photoactive monomer are compatible with each other. Use of a matrix precursor and photoactive monomer that polymerize by independent reactions substantially prevents cross-reaction between the photoactive monomer and the matrix precursor during the cure and inhibition of subsequent monomer polymerization. Use of a matrix precursor and photoactive monomer that result in compatible polymers substantially avoids phase separation. And in situ formation allows fabrication of articles with desirable thicknesses.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of application Ser. No.09/046,822, filed on Mar. 24, 1998, now U.S. Pat. No. 6,103,454.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to optical articles including holographicrecording media, in particular media useful either with holographicstorage systems or as components such as optical filters or beamsteerers.

2. Discussion of the Related Art

Developers of information storage devices and methods continue to seekincreased storage capacity. As part of this development, so-calledpage-wise memory systems, in particular holographic systems, have beensuggested as alternatives to conventional memory devices. Page-wisesystems involve the storage and readout of an entire two-dimensionalrepresentation, e.g., a page, of data. Typically, recording light passesthrough a two-dimensional array of dark and transparent areasrepresenting data, and the holographic system stores, in threedimensions, holographic representations of the pages as patterns ofvarying refractive index imprinted into a storage medium. Holographicsystems are discussed generally in D. Psaltis et al., “HolographicMemories,” Scientific American, November 1995, the disclosure of whichis hereby incorporated by reference. One method of holographic storageis phase correlation multiplex holography, which is described in U.S.Pat. No. 5,719,691 issued Feb. 17, 1998, the disclosure of which ishereby incorporated by reference. In one embodiment of phase correlationmultiplex holography, a reference light beam is passed through a phasemask, and intersected in the recording medium with a signal beam thathas passed through an array representing data, thereby forming ahologram in the medium. The spatial relation of the phase mask and thereference beam is adjusted for each successive page of data, therebymodulating the phase of the reference beam and allowing the data to bestored at overlapping areas in the medium. The data is laterreconstructed by passing a reference beam through the original storagelocation with the same phase modulation used during data storage. It isalso possible to use volume holograms as passive optical components tocontrol or modify light directed at the medium, e.g., filters or beamsteerers. Writing processes that provide refractive index changes arealso capable of forming articles such as waveguides.

FIG. 1 illustrates the basic components of a holographic system 10.System 10 contains a modulating device 12, a photorecording medium 14,and a sensor 16. Modulating device 12 is any device capable of opticallyrepresenting data in two-dimensions. Device 12 is typically a spatiallight modulator that is attached to an encoding unit which encodes dataonto the modulator. Based on the encoding, device 12 selectively passesor blocks portions of a signal beam 20 passing through device 12. Inthis manner, beam 20 is encoded with a data image. The image is storedby interfering the encoded signal beam 20 with a reference beam 22 at alocation on or within photorecording medium 14. The interference createsan interference pattern (or hologram) that is captured within medium 14as a pattern of, for example, varying refractive index. It is possiblefor more than one holographic image to be stored at a single location,or for holograms to be stored in overlapping positions, by, for example,varying the angle, the wavelength, or the phase of the reference beam22, depending on the particular reference beam employed. Signal beam 20typically passes through lens 30 before being intersected with referencebeam 22 in the medium 14. It is possible for reference beam 22 to passthrough lens 32 before this intersection. Once data is stored in medium14, it is possible to retrieve the data by intersecting reference beam22 with medium 14 at the same location and at the same angle,wavelength, or phase at which reference beam 22 was directed duringstorage of the data. The reconstructed data passes through lens 34 andis detected by sensor 16. Sensor 16 is, for example, a charged coupleddevice or an active pixel sensor. Sensor 16 typically is attached to aunit that decodes the data.

The capabilities of such holographic storage systems are limited in partby the storage media. Iron-doped lithium niobate has been used as astorage medium for research purposes for many years. However, lithiumniobate is expensive, exhibits poor sensitivity (1 J/cm²), has low indexcontrast (Δn of about 10⁻⁴), and exhibits destructive read-out (i.e.,images are destroyed upon reading). Alternatives have therefore beensought, 1o particularly in the area of photosensitive polymer films.See, e.g., W. K. Smothers et al., “Photopolymers for Holography,” SPIEOE/Laser Conference, 1212-03, Los Angeles, Calif., 1990. The materialdescribed in this article contains a photoimageable system containing aliquid monomer material (the photoactive monomer) and a photoinitiator(which promotes the polymerization of the monomer upon exposure tolight), where the photoimageable system is in an organic polymer hostmatrix that is substantially inert to the exposure light. During writingof information into the material (by passing recording light through anarray representing data), the monomer polymerizes in the exposedregions. Due to the lowering of the monomer concentration caused by thepolymerization, monomer from the dark, unexposed regions of the materialdiffuses to the exposed regions. The polymerization and resultingconcentration gradient create a refractive index change, forming thehologram representing the data. Unfortunately, deposition onto asubstrate of the pre-formed matrix material containing thephotoimageable system requires use of solvent, and the thickness of thematerial is therefore limited, e.g., to no more than about 150 μm, toallow enough evaporation of the solvent to attain a stable material andreduce void formation. In holographic processes such as described above,which utilize three-dimensional space of a medium, the storage capacityis proportional to a medium's thickness. Thus, the need for solventremoval inhibits the storage capacity of a medium. (Holography of thistype is typically referred to as volume holography because a Klein-CookQ parameter greater than 1 is exhibited (see W. Klein and B. Cook,“Unified approach to ultrasonic light diffraction,” IEEE Transaction onSonics and Ultrasonics, SU-14, 1967, at 123-134). In volume holography,the media thickness is generally greater than the fringe spacing,)

U.S. patent application Ser. No. 08/698,142 (our referenceColvin-Harris-Katz-Schilling 1-2-16-10), the disclosure of which ishereby incorporated by reference, also relates to a photoimageablesystem in an organic polymer matrix, but allows fabrication of thickermedia. In particular, the application discloses a recording mediumformed by polymerizing matrix material in situ from a fluid mixture oforganic oligomer matrix precursor and a photoimageable system. A similartype of system, but which does not incorporate oligomers, is discussedin D. J. Lougnot et al., Pure and Appl. Optics, 2, 383 (1993). Becauselittle or no solvent is typically required for deposition of thesematrix materials, greater thicknesses are possible, e.g., 200 μm andabove. However, while useful results are obtained by such processes, thepossibility exists for reaction between the precursors to the matrixpolymer and the photoactive monomer. Such reaction would reduce therefractive index contrast between the matrix and the polymerizedphotoactive monomer, thereby affecting to an extent the strength of thestored hologram.

Thus, while progress has been made in fabricating photorecording mediasuitable for use in holographic storage systems, further progress isdesirable. In particular, media which are capable of being formed inrelatively thick layers, e.g., greater than 200 μm, which substantiallyavoid reaction between the matrix material and photomonomer, and whichexhibit useful holographic properties, are desired.

SUMMARY OF THE INVENTION

The invention constitutes an improvement over prior recording media. Theinvention's use of a matrix precursor (i.e., the one or more compoundsfrom which the matrix is formed) and a photoactive monomer thatpolymerize by independent reactions substantially prevents bothcross-reaction between the photoactive monomer and the matrix precursorduring the cure, and inhibition of subsequent monomer polymerization.Use of a matrix precursor and photoactive monomer that form compatiblepolymers substantially avoids phase separation. And in situ formationallows fabrication of media with desirable thicknesses. These materialproperties are also useful for forming a variety of optical articles(optical articles being articles that rely on the formation ofrefractive index patterns or modulations in the refractive index tocontrol or modify light that is directed at them). In addition torecording media, such articles include, but are not limited to, opticalwaveguides, beam steerers, and optical filters. Independent reactionsindicate: (a) the reactions proceed by different types of reactionintermediates, e.g., ionic vs. free radical, (b) neither theintermediate nor the conditions by which the matrix is polymerized willinduce substantial polymerization of the photoactive monomer functionalgroups, i.e., the group or groups on a photoactive monomer that are thereaction sites for polymerization during the pattern (e.g., hologram)writing process (substantial polymerization indicates polymerization ofmore than 20% of the monomer functional groups), and (c) neither theintermediate nor the conditions by which the matrix is polymerized willinduce a non-polymerization reaction of the monomer functional groupsthat either causes cross-reaction between monomer functional groups andthe matrix or inhibits later polymerization of the monomer functionalgroups. Polymers are considered to be compatible if a blend of thepolymers is characterized, in 90° light scattering of a wavelength usedfor hologram formation, by a Rayleigh ratio (R₉₀°) less than 7×10⁻³cm⁻¹. The Rayleigh ratio (R_(θ)) is a conventionally known property, andis defined as the energy scattered by a unit volume in the direction θ,per steradian, when a medium is illuminated with a unit intensity ofunpolarized light, as discussed in M. Kerker, The Scattering of Lightand Other Electromagnetic Radiation, Academic Press, San Diego, 1969, at38. The Rayleigh ratio is typically obtained by comparison to the energyscatter of a reference material having a known Rayleigh ratio. Polymerswhich are considered to be miscible, e.g., according to conventionaltests such as exhibition of a single glass transition temperature, willtypically be compatible as well. But polymers that are compatible willnot necessarily be miscible. In situ indicates that the matrix is curedin the presence of the photoimageable system. A useful photorecordingmaterial, i.e., the matrix material plus the photoactive monomer,photoinitiator, and/or other additives, is attained, the materialcapable of being formed in thicknesses greater than 200 μm,advantageously greater than 500 μm, and, upon flood exposure, exhibitinglight scattering properties such that the Rayleigh ratio, R₉₀, is lessthan 7×10³¹ ³. (Flood exposure is exposure of the entire photorecordingmaterial by incoherent light at wavelengths suitable to inducesubstantially complete polymerization of the photoactive monomerthroughout the material.)

The optical article of the invention is formed by steps including mixinga matrix precursor and a photoactive monomer, and curing the mixture toform the matrix in situ. As discussed previously, the reaction by whichthe matrix precursor is polymerized during the cure is independent fromthe reaction by which the photoactive monomer is later polymerizedduring writing of a pattern, e.g., data or waveguide form, and, inaddition, the matrix polymer and the polymer resulting frompolymerization of the photoactive monomer (hereafter referred to as thephotopolymer) are compatible with each other. (The matrix is consideredto be formed when the photorecording material exhibits an elasticmodulus of at least about 10⁵ Pa. Curing indicates reacting the matrixprecursor such that the matrix provides this elastic modulus in thephotorecording material.) The optical article of the invention containsa three-dimensional crosslinked polymer matrix and one or morephotoactive monomers. At least one photoactive monomer contains one ormore moieties, excluding the monomer functional groups, that aresubstantially absent from the polymer matrix. (Substantially absentindicates that it is possible to find a moiety in the photoactivemonomer such that no more than 20% of all such moieties in thephotorecording material are present, i.e., covalently bonded, in thematrix.) The resulting independence between the host matrix and themonomer offers useful recording properties in holographic media anddesirable properties in waveguides such as enabling formation of largemodulations in the refractive index without the need for highconcentrations of the photoactive monomer. Moreover, it is possible toform the material of the invention without the disadvantageous solventdevelopment required previously.

In contrast to a holographic recording medium of the invention, mediawhich utilize a matrix precursor and photoactive monomer that polymerizeby non-independent reactions often experience substantial cross-reactionbetween the precursor and the photoactive monomer during the matrix cure(e.g., greater than 20% of the monomer is attached to the matrix aftercure), or other reactions that inhibit polymerization of the photoactivemonomer. Cross-reaction tends to undesirably reduce the refractive indexcontrast between the matrix and the photoactive monomer and is capableof affecting the subsequent polymerization of the photoactive monomer,and inhibition of monomer polymerization clearly affects the process ofwriting holograms. As for compatibility, previous work has beenconcerned with the compatibility of the photoactive monomer in a matrixpolymer, not the compatibility of the resulting photopolymer in thematrix. Yet, where the photopolymer and matrix polymer are notcompatible, phase separation typically occurs during hologram formation.It is possible for such phase separation to lead to increased lightscattering, reflected in haziness or opacity, thereby degrading thequality of the medium, and the fidelity with which stored data iscapable of being recovered

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a basic holographic storage system.

FIG. 2 shows the effect of several different photoactive monomers on arecording medium's refractive index contrast.

DETAILED DESCRIPTION OF THE INVENTION

The optical article, e.g., holographic recording medium, of theinvention is formed by steps including mixing a matrix precursor and aphotoactive monomer, and curing the mixture to form the matrix in situ.The matrix precursor and photoactive monomer are selected such that (a)the reaction by which the matrix precursor is polymerized during thecure is independent from the reaction by which the photoactive monomerwill be polymerized during writing of a pattern, e.g., data, and (b) thematrix polymer and the polymer resulting from polymerization of thephotoactive monomer (the photopolymer) are compatible with each other.As discussed previously, the matrix is considered to be formed when thephotorecording material, i.e., the matrix material plus the photoactivemonomer, photoinitiator, and/or other additives, exhibits an elasticmodulus of at least about 10⁵ Pa, generally about 10⁵ Pa to about 10⁹Pa, advantageously about 10⁶ Pa to about 10⁸ Pa.

The compatibility of the matrix polymer and photopolymer tends toprevent large-scale (>100 nm) phase separation of the components, suchlarge-scale phase separation typically leading to undesirable hazinessor opacity. Utilization of a photoactive monomer and a matrix precursorthat polymerize by independent reactions provides a cured matrixsubstantially free of cross-reaction, i.e., the photoactive monomerremains substantially inert during the matrix cure. In addition, due tothe independent reactions, there is no inhibition of subsequentpolymerization of the photoactive monomer. At least one photoactivemonomer contains one or more moieties, excluding the monomer functionalgroups, that are substantially absent from the polymer matrix, i.e., itis possible to find a moiety in the photoactive monomer such that nomore than 20% of all such moieties in the photorecording material arepresent, i.e., covalently bonded, in the matrix. The resulting opticalarticle is capable of exhibiting desirable refractive index contrast dueto the independence of the matrix from the photoactive monomer.

As discussed above, formation of a hologram, waveguide, or other opticalarticle relies on a refractive index contrast (Δn) between exposed andunexposed regions of a medium, this contrast at least partly due tomonomer diffusion to exposed regions. High index contrast is desiredbecause it provides improved signal strength when reading a hologram,and provides efficient confinement of an optical wave in a waveguide.One way to provide high index contrast in the invention is to use aphotoactive monomer having moieties (referred to as index-contrastingmoieties) that are substantially absent from the matrix, and thatexhibit a refractive index substantially different from the indexexhibited by the bulk of the matrix. For example, high contrast would beobtained by using a matrix that contains primarily aliphatic orsaturated alicyclic moieties with a low concentration of heavy atoms andconjugated double bonds (providing low index) and a photoactive monomermade up primarily of aromatic or similar high-index moieties.

The matrix is a solid polymer formed in situ from a matrix precursor bya curing step (curing indicating a step of inducing reaction of theprecursor to form the polymeric matrix). It is possible for theprecursor to be one or more monomers, one or more oligomers, or amixture of monomer and oligomer. In addition, it is possible for thereto be greater than one type of precursor functional group, either on asingle precursor molecule or in a group of precursor molecules.(Precursor functional groups are the group or groups on a precursormolecule that are the reaction sites for polymerization during matrixcure.) To promote mixing with the photoactive monomer, the precursor isadvantageously liquid at some temperature between about −50° C. andabout 80° C. Advantageously, the matrix polymerization is capable ofbeing performed at room temperature. Also advantageously, thepolymerization is capable of being performed in a time period less than300 minutes, advantageously 5 to 200 minutes. The glass transitiontemperature (T_(g)) of the photorecording material is advantageously lowenough to permit sufficient diffusion and chemical reaction of thephotoactive monomer during a holographic recording process. Generally,the T_(g) is not more than 50° C. above the temperature at whichholographic recording is performed, which, for typical holographicrecording, means a T_(g) between about 80° C. and about −130° C. (asmeasured by conventional methods). It is also advantageous for thematrix to exhibit a three-dimensional network structure, as opposed to alinear structure, to provide the desired modulus discussed previously.

Examples of polymerization reactions contemplated for forming matrixpolymers in the invention include cationic epoxy polymerization,cationic vinyl ether polymerization, cationic alkenyl etherpolymerization, cationic allene ether polymerization, cationic keteneacetal polymerization, epoxy-amine step polymerization, epoxy-mercaptanstep polymerization, unsaturated ester-amine step polymerization (viaMichael addition), unsaturated ester-mercaptan step polymerization (viaMichael addition), vinyl-silicon hydride step polymerization(hydrosilylation), isocyanate-hydroxyl step polymerization (urethaneformation), and isocyanate-amine step polymerization (urea formation).

Several such reactions are enabled or accelerated by suitable catalysts.For example, cationic epoxy polymerization takes place rapidly at roomtemperature by use of BF₃-based catalysts, other cationicpolymerizations proceed in the presence of protons, epoxy-mercaptanreactions and Michael additions are accelerated by bases such as amines,hydrosilylation proceeds rapidly in the presence of transition metalcatalysts such as platinum, and urethane and urea formation proceedrapidly when tin catalysts are employed. It is also possible to usephotogenerated catalysts for matrix formation, provided that steps aretaken to prevent polymerization of the photoactive monomer during thephotogeneration.

The photoactive monomer is any monomer or monomers capable of undergoingphotoinitiated polymerization, and which, in combination with a matrixmaterial, meets the polymerization reaction and compatibilityrequirements of the invention. Suitable photoactive monomers includethose which polymerize by a free-radical reaction, e.g., moleculescontaining ethylenic unsaturation such as acrylates, methacrylates,acrylamides, methacrylamides, styrene, substituted styrenes, vinylnaphthalene, substituted vinyl naphthalenes, and other vinylderivatives. Free-radical copolymerizable pair systems such as vinylether mixed with maleate and thiol mixed with olefin are also suitable.It is also possible to use cationically polymerizable systems such asvinyl ethers, alkenyl ethers, allene ethers, ketene acetals, andepoxies. It is also possible for a single photoactive monomer moleculeto contain more than one monomer functional group.

As mentioned previously, relatively high index contrast is desired inthe article of the invention, whether for improved readout in arecording media or efficient light confinement in a waveguide. Inaddition, it is advantageous to induce this relatively large indexchange with a small number of monomer functional groups, becausepolymerization of the monomer generally induces shrinkage in a material.(For instance, in Examples 2 and 3 below, the writing-induced shrinkagedue to hologram recording was approximately 0.7%. Example 4 reflects areduced concentration of photoactive monomer, and an associatedreduction in writing-induced shrinkage (0.30 to 0.35%).) Such shrinkagehas a detrimental effect on the retrieval of data from stored holograms,and also degrades the performance of waveguide devices such as byincreased transmission losses or other performance deviations. Loweringthe number of monomer functional groups that must be polymerized toattain the necessary index contrast is therefore desirable. Thislowering is possible by increasing the ratio of the molecular volume ofthe monomers to the number of monomer functional groups on the monomers.This increase is attainable by incorporating into a monomer largerindex-contrasting moieties and/or a larger number of index-contrastingmoieties. For example, if the matrix is composed primarily of aliphaticor other low index moieties and the monomer is a higher index specieswhere the higher index is imparted by a benzene ring, the molecularvolume could be increased relative to the number of monomer functionalgroups by incorporating a naphthalene ring instead of a benzene ring(the naphthalene having a larger volume), or by incorporating one ormore additional benzene rings, without increasing the number of monomerfunctional groups. In this manner, polymerization of a given volumefraction of the monomers with the larger molecular volume/monomerfunctional group ratio would require polymerization of less monomerfunctional groups, thereby inducing less shrinkage. But the requisitevolume fraction of monomer would still diffuse from the unexposed regionto the exposed region, providing the desired refractive index.

The molecular volume of the monomer, however, should not be so large asto slow diffusion below an acceptable rate. Diffusion rates arecontrolled by factors including size of diffusing species, viscosity ofthe medium, and intermolecular interactions. Larger species tend todiffuse more slowly, but it would be possible in some situations tolower the viscosity or make adjustments to the other molecules presentin order to raise diffusion to an acceptable level. Also, in accord withthe discussion herein, it is important to ensure that larger moleculesmaintain compatibility with the matrix.

Numerous architectures are possible for monomers containing multipleindex-contrasting moieties. For example, it is possible for the moietiesto be in the main chain of a linear oligomer, or to be substituentsalong an oligomer chain. Alternatively, it is possible for theindex-contrasting moieties to be the subunits of a branched or dendriticlow molecular weight polymer.

In addition to the photoactive monomer, the optical article typicallycontains a photoinitiator (the photoinitiator and: photoactive monomerbeing part of the overall photoimageable system). The photoinitiator,upon exposure to relatively low levels of the recording light,chemically initiates the polymerization of the monomer, avoiding theneed for direct light-induced polymerization of the monomer. Thephotoinitiator generally should offer a source of species that initiatepolymerization of the particular photoactive monomer. Typically, 0.1 to20 wt. % photoinitiator, based on the weight of the photoimageablesystem, provides desirable results.

A variety of photoinitiators known to those skilled in the art andavailable commercially are suitable for use in the invention. It isadvantageous to use a photoinitiator that is sensitive to light in thevisible part of the spectrum, particularly at wavelengths available fromconventional laser sources, e.g., the blue and green lines of Ar⁺ (458,488, 514 nm) and He—Cd lasers (442 nm), the green line of frequencydoubled YAG lasers (532 nm), and the red lines of He—Ne (633 nm) andKr⁺lasers (647 and 676 nm). One advantageous free radical photoinitiatoris bis(η-5-2,4-cyclopentadien-1-yl)bis[2,6-difluoro-3-(1H-pyrrol-1-yl)phenyl]titanium, available commerciallyfrom Ciba as CGI-784. Another visible free-radical photoinitiator (whichrequires a co-initiator) is 5,7,diiodo-3-butoxy-6-fluorone, commerciallyavailable from Spectra Group Limited as H-Nu 470. Free-radicalphotoinitiators of dye-hydrogen donor systems are also possible.Examples of suitable dyes include eosin, rose bengal, erythrosine, andmethylene blue, and suitable hydrogen donors include tertiary aminessuch as n-methyl diethanol amine. In the case of cationicallypolymerizable monomers, a cationic photoinitiator is used, such as asulfonium salt or an iodonium salt. These cationic photoinitiator saltsabsorb predominantly in the UV portion of the spectrum, and aretherefore typically sensitized with a dye to allow use of the visibleportion of the spectrum. An example of an alternative visible cationicphotoinitiator is (η₅-2,4-cyclopentadien-1-yl)(η₆-isopropylbenzene)-iron(II) hexafluorophosphate, available commercialfrom Ciba as Irgacure 261. It is also conceivable to use other additivesin the photoimageable system, e.g., inert diffusing agents havingrelatively high or low refractive indices.

Advantageously, for holographic recording, the matrix is a polymer,formed by mercaptan-epoxy step polymerization, more advantageously apolymer formed by mercaptan-epoxy step polymerization having a polyetherbackbone. The polyether backbone offers desirable compatibility withseveral useful photoactive monomers, particularly vinyl aromaticcompounds. Specifically, photoactive monomers selected from styrene,bromostyrene, divinyl benzene, and 4-methylthio-1-vinylnaphthalene(MTVN) have been found to be useful with matrix polymers formed. bymercaptan-epoxy step polymerization and having a polyether backbone. Amonomer that has more than one index-contrasting moiety and that is alsouseful with these polyether matrix polymers is1-(3-(naphth-1-ylthio)propylthio)-4-vinylnaphthalene.

To be independent, the polymerization reactions for the matrix precursorand the photoactive monomer are selected such that: (a) the reactionsproceed by different types of reaction intermediates, (b) neither theintermediate nor the conditions by which the matrix is polymerized willinduce substantial polymerization of the photoactive monomer functionalgroups, and (c) neither the intermediate nor the conditions by which thematrix is polymerized will induce a non-polymerization reaction of themonomer functional groups that causes cross-reaction (between themonomer functional groups and the matrix polymer) or inhibits laterpolymerization of the monomer functional groups. According to item (a),if a matrix is polymerized by use of an ionic intermediate, it would besuitable to polymerize the photoactive monomer by use of a free radicalreaction. In accordance with item (b), however, the ionic intermediateshould not induce substantial polymerization of the photoactive monomerfunctional groups. Also in accordance with item (b), for example, onemust be aware that a photoinitiated free radical matrix polymerizationwill typically induce a photoinitiated cationic polymerization of aphotoactive monomer functional group. Thus, two otherwise independentreactions are not independent for purposes of the invention if both aredriven by a single reaction condition. In accordance with item (c), forexample, base-catalyzed matrix polymerization should not be performedwhen the photoactive monomer functional group undergoes anon-polymerization reaction in response to the base, even ifpolymerization of the monomer functional group is performed by anindependent reaction. A specific example is that a base-catalyzedepoxy-mercaptan polymerization should not be used with an acrylatemonomer because, although the acrylate is polymerized by a free radicalreaction, the acrylate will react with the mercaptans under basecatalysis, resulting in a cross-reaction.

Table I below illustrates some examples of matrix/photoactive monomercombinations where the matrix polymerization reaction and photoactivemonomer polymerization are capable of being independent, and exampleswhere the polymerizations interfere with each other. (Photoactivemonomers are horizontal, and matrix polymers are vertical. “X” indicatescross-reaction or monomer polymerization during matrix polymerization.“O” indicates independent reactions. “I” indicates that the photoactivemonomer polymerization is inhibited by the reagents or reaction thatform the polymeric matrix, e.g., the photoactive monomer functionalgroup is converted to a non-polymerizing group, or chemical species arepresent after the matrix cure that substantially slow the rate or yieldof polymerization of the monomer functional groups.)

Photoactive (Meth) Styrene Vinyl Matrix acrylates Derivatives EthersEpoxies Cationic Epoxy ◯ ◯ X X Cationic Vinyl ◯ ◯ X X Ethers Epoxy(amine) X ◯ I X Epoxy (mercaptan) X ◯ I X Unsaturated ester X ◯ I X(amine) Unsaturated ester X ◯ I X (mercaptan) Hydrosilylation X X X ◯Urethane formation ◯ ◯ ◯ X

For purposes of the invention, polymers are considered to be compatibleif a blend of the polymers is characterized, in 90° light scattering, bya Rayleigh ratio (R₉₀°) less than 7×10⁻³ cm⁻¹. The Rayleigh ratio,R_(θ), is a conventionally known property, and is defined as the energyscattered by a unit volume in the direction θ, per steradian, when amedium is illuminated with a unit intensity of unpolarized light, asdiscussed in M. Kerker, The Scattering of Light and OtherElectromagnetic Radiation, Academic Press, San Diego, 1969. The lightsource used for the measurement is generally a laser having a wavelengthin the visible part of the spectrum. Normally, the wavelength intendedfor use in writing holograms is used. The scattering measurements aremade upon a photorecording material that has been flood exposed. Thescattered light is collected at an angle of 90° from the incident light,typically by a photodetector. It is possible to place a narrowbandfilter, centered at the laser wavelength, in front of such aphotodetector to block fluorescent light, although such a step is notrequired. The Rayleigh ratio is typically obtained by comparison to theenergy scatter of a reference material having a known Rayleigh ratio.

Polymer blends which are considered to be miscible, e.g., according toconventional tests such as exhibition of a single glass transitiontemperature, will typically be compatible as well, i.e., miscibility isa subset of compatibility. Standard miscibility guidelines and tablesare therefore useful in selecting a compatible blend. However, it ispossible for polymer blends that are immiscible to be compatibleaccording to the light scattering test above.

A polymer blend is generally considered to be miscible if the blendexhibits a single glass transition temperature, T_(g), as measured byconventional methods. An immiscible blend will typically exhibit twoglass transition temperatures corresponding to the T_(g) values of theindividual polymers. T_(g) testing is most commonly performed bydifferential scanning calorimetry (DSC), which shows the T_(g) as a stepchange in the heat flow (typically the ordinate). The reported T_(g) istypically the temperature at which the ordinate reaches the mid-pointbetween extrapolated baselines before and after the transition. It isalso possible to use Dynamic Mechanical Analysis (DMA) to measure T_(g).DMA measures the storage modulus of a material, which drops severalorders of magnitude in the glass transition region. It is possible incertain cases for the polymers of a blend to have individual T_(g)values that are close to each other. In such cases, conventional methodsfor resolving such overlapping T_(g) should be used, such as discussedin Brinke et al., “The thermal characterization of multi-componentsystems by enthalpy relaxation,” Thermochimica Acta., 238 (1994), at 75.

Matrix polymer and photopolymer that exhibit miscibility are capable ofbeing selected in several ways. For example, several publishedcompilations of miscible polymers are available, such as O. Olabisi etal., Polymer-Polymer Miscibility, Academic Press, New York, 1979; L. M.Robeson, MMI. Press Symp. Ser., 2, 177, 1982; L. A. Utracki, PolymerAlloys and Blends: Thermodynamics and Rheology, Hanser Publishers,Munich, 1989; and S. Krause in Polymer Handbook, J. Brandrup and E. H.Immergut, Eds.; 3rd Ed., Wiley Interscience, New York, 1989, pp. VI347-370, the disclosures of which are hereby incorporated by reference.Even if a particular polymer of interest is not found in suchreferences, the approach specified allows determination of a compatiblephotorecording material by employing a control sample.

Determination of miscible or compatible blends is further aided byintermolecular interaction considerations that typically drivemiscibility. For example, it is well known that polystyrene andpoly(methylvinylether) are miscible because of an attractive interactionbetween the methyl ether group and the phenyl ring. It is thereforepossible to promote miscibility, or at least compatibility, of twopolymers by using a methyl ether group in one polymer and a phenyl groupin the other polymer. It has also been demonstrated that immisciblepolymers are capable of being made miscible by the incorporation ofappropriate functional groups that can provide ionic interactions. (SeeZ. L. Zhou and A. Eisenberg, J. Polym. Sci., Polym. Phys. Ed., 21 (4),595, 1983; R. Murali and A. Eisenberg, J. Polym. Sci., Part B: Polym.Phys., 26 (7), 1385, 1988; and A Natansohn et al., Makromol. Chem.,Macromol. Symp., 16, 175, 1988.) For example, polyisoprene andpolystyrene are immiscible. However, when polyisoprene is partiallysulfonated (5%), and 4-vinyl pyridine is copolymerized with thepolystyrene, the blend of these two functionalized polymers is miscible.It is contemplated that the ionic interaction between the sulfonatedgroups and the pyridine group (proton transfer) is the driving forcethat makes this blend miscible. Similarly, polystyrene and poly(ethylacrylate), which are normally immiscible, have been made miscible bylightly sulfonating the polystyrene. (See R. E. Taylor-Smith and R. A.Register, Macromolecules, 26, 2802, 1993.) Charge-transfer has also beenused to make miscible polymers that are otherwise immiscible. Forexample it has been demonstrated that, although poly(methyl acrylate)and poly(methyl methacrylate) are immiscible, blends in which the formeris copolymerized with (N-ethylcarbazol-3-yl)methyl acrylate (electrondonor) and the latter is copolymerized with2-[(3,5-dinitrobenzoyl)oxy]ethyl methacrylate (electron accceptor) aremiscible, provided the right amounts of donor and acceptor are used.(See M. C. Piton and A. Natansohn, Macromolecules, 28, 15, 1995.)Poly(methyl methacrylate) and polystyrene are also capable of being mademiscible using the corresponding donor-acceptor co-monomers (See M. C.Piton and A. Natansohn, Macromolecules, 28, 1605, 1995).

A variety of test methods exist for evaluating the miscibility orcompatibility of polymers, as reflected in the recent overview publishedin A. Hale and H. Bair, Ch. 4—“Polymer Blends and Block Copolymers,”Thermal Characterization of Polymeric Materials, 2nd Ed., AcademicPress, 1997. For example, in the realm of optical methods, opacitytypically indicates a two-phase material, whereas clarity generallyindicates a compatible system. Other methods for evaluating miscibilityinclude neutron scattering, infrared spectroscopy (IR), nuclear magneticresonance (NMR), x-ray scattering and diffraction, fluorescence,Brillouin scattering, melt titration, calorimetry, andchemilluminescence. See, for example, L. Robeson, supra; S. Krause,Chemtracts—Macromol. Chem., 2, 367, 1991a; D. Vesely in Polymer Blendsand Alloys, M. J. Folkes and P. S. Hope, Eds., Blackie Academic andProfessional, Glasgow, pp. 103-125; M. M. Coleman et al. SpecificInteractions and the Miscibility of Polymer Blends, TechnomicPublishing, Lancaster, Pa., 1991; A. Garton, Infrared Spectroscopy ofPolymer Blends Composites and Surfaces, Hanser, New York, 1992; L. W.Kelts et al., Macromolecules, 26, 2941, 1993; and J. L. White and P. A.Mirau, Macromolecules, 26, 3049, 1993; J. L. White and P. A. Mirau,Macromolecules, 27, 1648, 1994; and C. A. Cruz et al., Macromolecules,12, 726, 1979; and C. J. Landry et al., Macromolecules, 26, 35, 1993.

Compatibility has also been promoted in otherwise incompatible polymersby incorporating reactive groups into the polymer matrix, where suchgroups are capable of reacting with the photoactive monomer during theholographic recording step. Some of the photoactive monomer will therebybe grafted onto the matrix during recording. If there are enough ofthese grafts, it is possible to prevent or reduce phase separationduring recording. However, if the refractive index of the grafted moietyand of the monomer are relatively similar, too many grafts, e.g., morethan 30% of monomers grafted to the matrix, will tend to undesirablyreduce refractive index contrast.

A holographic recording medium of the invention is formed by adequatelysupporting the photorecording material, such that holographic writingand reading is possible. Typically, fabrication of the medium involvesdepositing the matrix precursor/photoimageable system mixture betweentwo plates using, for example, a gasket to contain the mixture. Theplates are typically glass, but it is also. possible to use othermaterials transparent to the radiation used to write data, e.g., aplastic such as polycarbonate or poly(methyl methacrylate). It ispossible to use spacers between the plates to maintain a desiredthickness for the recording medium. During the matrix cure, it ispossible for shrinkage in the material to create stress in the plates,such stress altering the parallelism and/or spacing of the plates andthereby detrimentally affecting the medium's optical properties. Toreduce such effects, it is useful to place the plates in an apparatuscontaining mounts, e.g., vacuum chucks, capable of being adjusted inresponse to changes in parallelism and/or spacing. In such an apparatus,it is possible to monitor the parallelism in real-time by use of aconventional interferometric method, and make any necessary adjustmentsduring the cure. Such a method is discussed, for example, in U.S. patentapplication Ser. No. 08/867,563 (our reference Campbell-Harris-Levinos3-5-3), the disclosure of which is hereby incorporated by reference. Thephotorecording material of the invention is also capable of beingsupported in other ways. For instance, it is conceivable to dispose thematrix precursor/photoimageable system mixture into the pores of asubstrate, e.g., a nanoporous glass material such as Vycor, prior tomatrix cure. More conventional polymer processing is also envisioned,e.g., closed mold formation or sheet extrusion. A stratified medium isalso contemplated, i.e., a medium containing multiple substrates, e.g.,glass, with layers of photorecording material disposed between thesubstrates.

The medium of the invention is then capable of being used in aholographic system such as discussed previously. The amount ofinformation capable of being stored in a holographic medium isproportional to the product of: the refractive index contrast, Δn, ofthe photorecording material, and the thickness, d, of the photorecordingmaterial. (The refractive index contrast, Δn, is conventionally known,and is defined as the amplitude of the sinusoidal variations in therefractive index of a material in which a plane-wave, volume hologramhas been written. The refractive index varies as: n(x)=n₀+Δn cos(K_(x)),where n(x) is the spatially varying refractive index, x is the positionvector, K is the grating wavevector, and n₀ is the baseline refractiveindex of the medium. See, e.g., P. Hariharan, Optical Holography:Principles, Techniques and Applications, Cambridge University Press,Cambridge, 1991, at 44.) The Δn of a material is typically calculatedfrom the diffraction efficiency or efficiencies of a single volumehologram or a multiplexed set of volume holograms recorded in a medium.The Δn is associated with a medium before writing, but is observed bymeasurement performed after recording. Advantageously, thephotorecording material of the invention exhibits a Δn of 3×10⁻³ orhigher:

Examples of other optical articles include beam filters, beam steerersor deflectors, and optical couplers. (See, e.g., L. Solymar and D.Cooke, Volume Holography and Volume Gratings, Academic Press, 315-327(1981), the disclosure of which is hereby incorporated by reference.) Abeam filter separates part of an incident laser beam that is travelingalong a particular angle from the rest of the beam. Specifically, theBragg selectivity of a thick transmission hologram is able toselectively diffract light along a particular angle of incidence, whilelight along other angle travels undefiected through the hologram. (See,e.g., J. E. Ludman et al., “Very thick holographic nonspatial filteringof laser beams,” Optical Engineering, Vol. 36, No. 6, 1700 (1997), thedisclosure of which is hereby incorporated by reference.) A beam steereris a hologram that deflects light incident at the Bragg angle. Anoptical coupler is typically a combination of beam deflectors that steerlight from a source to a target. These articles, typically referred toas holographic optical elements, are fabricated by imaging a particularoptical interference pattern within a recording medium, as discussedpreviously with respect to data storage. Medium for these holographicoptical elements are capable of being formed by the techniques discussedherein for recording media or waveguides.

As mentioned previously, the materials principles discussed herein areapplicable not only to hologram formation, but also to formation ofoptical transmission devices such as waveguides. Polymeric opticalwaveguides are discussed for example in B. L. Booth, “OpticalInterconnection Polymers,” in Polymers for Lightwave and IntegratedOptics, Technology and Applications, L. A. Hornak, ed., Marcel Dekker,Inc. (1992); U.S. Pat. No. 5,292,620; and U.S. Pat. No. 5,219,710, thedisclosures of which are hereby incorporated by reference. Essentially,the recording material of the invention is irradiated in a desiredwaveguide pattern to provide refractive index contrast between thewaveguide pattern and the surrounding (cladding) material. It ispossible for exposure to be performed, for example, by a focused laserlight or by use of a mask with a non-focused light source. Generally, asingle layer is exposed in this manner to provide the waveguide pattern,and additional layers are added to complete the cladding, therebycompleting the waveguide. This process is discussed for example at pages235-36 of Booth, supra, and Cols. 5 and 6 of U.S. Pat. No. 5,292,620. Abenefit of the invention is that by using conventional moldingtechniques, it is possible to mold the matrix/photoimageable systemmixture into a variety of shapes prior to matrix cure. For example, thematrix/photoimageable system mixture is able to be molded into ridgewaveguides, wherein refractive index patterns are then written into themolded structures. It is thereby possible to easily form structures suchas Bragg gratings. This feature of the invention increases the breadthof applications in which such polymeric waveguides would be useful.

The invention will be further clarified by the following examples, whichare intended to be exemplary.

COMPARATIVE EXAMPLE 1

A solution was prepared containing 89.25 wt. % phenoxyethyl acrylate(photoactive monomer), 10.11 wt. % ethoxylated bisphenol-A diacrylate(photoactive monomer), 0.5 wt. % Ciba CGI-784 (identified previously)(photoinitiator), and 0.14 wt. % dibutyltin dilaurate (catalyst formatrix formation). 0.0904 g of the solution was added to a vialcontaining 0.2784 g diisocyanate-terminated polypropylene glycol(MW=2471) (matrix precursor) and 0.05 g α,ω-dihydroxypolypropyleneglycol (MW=425) (matrix precursor). The mixture was thoroughly mixed andallowed to polymerize overnight at room temperature, while protectedfrom light. The polymerization was a step polymerization of theisocyanate groups with the hydroxyl groups to form a polyurethane withdissolved acrylate monomers. The mixture appeared clear and transparentto the naked eye. Upon exposure to an intense tungsten light, whichinitiated polymerization of the acrylate monomers, the material turnedmilky white, indicating that the polyurethane matrix and acrylatepolymers were not compatible.

Consultation of the polymer miscibility table published by Krause,referenced above, shows that polyurethanes are miscible, and thuscompatible, with Saran®, a chlorinated polymer. Example 1 reflects asystem made using this information.

EXAMPLE 1

A solution was prepared containing 98.86 wt. % 4-chlorophenyl acrylateand 1.14 wt. % dibutyltin dilaurate. 0.017 g of this solution was addedto a vial containing 0.2519 g diisocyanate-terminated polypropyleneglycol (MW=2471), 0.047 g α,ω-dihydroxypolypropylene glycol (MW=425),0.051 g 4-chlorophenyl acrylate, and 0.00063 g Ciba CGI-784(photoinitiator). The mixture was thoroughly mixed and allowed to cureovernight at room temperature, while protected from light. Thepolymerization was a step polymerization of the isocyanate groups withthe hydroxyl groups to form a polyurethane with dissolved chlorophenylacrylate monomer. The mixture appeared clear and transparent to thenaked eye. Upon exposure to an intense tungsten light, which initiatedpolymerization of the acrylate monomer, the sample remained clear andtransparent, indicating the compatibility of the monomer and the matrixpolymer.

EXAMPLE 2

0.00265 g Ciba CGI-784 was dissolved in 0.26248 g styrene (photoactivemonomer). The solution was mixed-with 1.9187 g polypropyleneglycoldiglycidyl ether (MW=380) (PPGDGE) (matrix precursor), 1.2428 gpentaerythritoltetrakis(mercaptopropionate) (PETMP) (matrix precursor),and 0.1445 g tris(2,4,6-dimethylaminomethyl)phenol (TDMAMP) (catalystfor matrix formation). The:mixture was dispensed on a glass slide, intoan approximately 200 μm thick, 25 mm diameter Teflon spacer, and secondglass slide was placed thereon. After about one hour at roomtemperature, the mixture gelled due to the amine-catalyzedcopolymerization of the mercaptan with the epoxy. Differential scanningcalorimeter (DSC) and Fourier transform infrared (FTIR) measurementsindicated that polymerization of the matrix was complete after two hours(i.e., no measurable amount of precursor functional groups). A tough,elastomeric material was obtained, consisting of an epoxy-mercaptanmatrix containing dissolved styrene monomer and photoinitiator. Thethickness of the medium was about 270 to 290 μm. After 24 hours, aseries of multiplexed holograms were written into the medium, inaccordance with the procedure of U.S. Pat. No. 5,719,691, referencedpreviously. A Δn of 1.7×10⁻³ was achieved. No abnormal light scatteringwas detected after holographic recording, indicating compatibilitybetween the polymerized styrene monomer and the epoxy-mercaptan matrix.

EXAMPLE 3

To increase the Δn of the medium above that produced from Example 2,bromostyrene monomer was used as the photoactive monomer. 0.01262 g CibaCGI-784 was dissolved in 0.2194 g 4-bromostyrene (photoactive monomer).The solution was mixed with 0.9597 g PPGDGE, 0.6042 g PETMP, and 0.084 gTDMAMP. Samples were prepared and holograms recorded as in Example 2. Anaverage Δn of 4.2×10⁻³ was attained. Again, no abnormal light scatteringwas detected after holographic recording, and, in addition, DSC. showedonly one glass transition temperature, suggesting a compatible system.

EXAMPLE 4

0.054 g Ciba CGI-784 was dissolved in 0.46 g 4-bromostyrene. Thesolution was mixed with 3.8 g PPGDGE, 2.44 g PETMP, and 0.3 g TDMAMP.This corresponds to half the concentration of bromostyrene used inExample 3. Samples were prepared and holograms recorded as in Example 2.A Δn of 2.5×10⁻³ was attained. The decrease in thickness (shrinkage)induced by polymerization of the bromostyrene was about 0.3%. Theelastic modulus of the photorecording material was about 5.7×10⁶ Pa.

EXAMPLE 5

4-methylthio-1-vinylnaphthalene (MTVN); (photoactive monomer) wassynthesized by the following procedure: 1-methylthionaphthalenepreparation: 63. g (0.25 mol) of 1-iodonaphthalene was dissolved in 1 Lof anhydrous ether under nitrogen. The solution was cooled to −70° C.,and 109 mL of 2.5 M butyllithium (BuLi) in hexane (0.27 mol) was addedover 30 min. with stirring. 25 g (0.27 mol) of dimethyl disulfide wasadded and the solution was allowed to warm to room temperature over 4hours. 200 mL of concentrated aqueous Na₂CO₃ was added and the organiclayer was dried with MgSO₄, filtered, and then concentrated to a darkorange oil containing 42 g (97%) of product and also containing about 10g of butyl iodide by-product. All glassware and other apparatus wascleaned with bleach to decompose the residual sulfides.

4-methylthio-1-naphthaldehyde preparation. 14.5 g of1-methylthionaphthalene (0.083 mol) was mixed with 12.4 g (0.17 mol) ofanhydrous N,N-dimethylformamide and the solution cooled with an icebath. 23.9 g (0.095 mol) of diphosphoryl tetrachloride was addeddropwise with stirring, keeping the temperature below 15° C. The mixturewas slowly heated to 100° C. and continually stirred at that temperaturefor 2 hours. The mixture was allowed to cool and was then chilled withan ice bath. A solution of 23 g of NaOH in 200 mL of water (cooled byadding 100 g of ice) was poured into the reaction mixture, and thecombined mixtures were gently warmed to 40° C. with stirring. At thispoint, an exothermic reaction began, the heat-was removed, and more icewas added to keep the temperature below 50° C. When the temperaturestabilized below 35° C., 200 mL of ether was added with stirring. Theorganic layer was separated and the aqueous layer extracted with 100additional mnL of ether. The combined ether extracts were dried withMgSO₄, filtered, concentrated, and chromatographed on a column of 120 gof silica gel, eluting with 500 mL portions of hexane containing 0, 25,50, and 75 volume % of CH₂Cl₂ and collecting 100 mL fractions. Theproduct was collected from 6-8 of these fractions, yielding 9.8 g (58%)of yellow solid. 4-methylthio-1-vinylnaphthalene preparation: Asuspension of 19.9 g (0.058 mol) of methyltriphenylphosphonium bromidepowder in 150 mL of anhydrous tetrahydrofuran was cooled to 0° C. withstirring under nitrogen. 19 mL of 2.5 M BuLi in hexane (0.48 mol) wasadded over 30 minutes, keeping the color as light as possible andavoiding a dark orange coloration. The mixture was allowed to warm to25° C., stirred 1 hour at that temperature, and cooled to 0° C. 9.8 g of4-methylthio-1-naphthaldehyde dissolved in 20 mL of tetrahydrofuran wasadded over 30 minutes, maintaining the stirring at 0° C. The mixture wasstirred overnight, allowing the temperature to rise to ambient. 10 mL ofmethanol was added, and the solvents evaporated at reduced pressure. Theresidue was extracted with 5 portions of 100 mL of ligroin (mainlyheptanes) at its boiling point of 90-110° C., keeping the residue softby adding additional methanol. The extracts were filtered, concentrated,and eluted through 40 g of silica gel with hexane. Product was obtainedfrom a 500 mL fraction, yielding 6.8 g pale yellow liquid (70%) whichwas stored at −20° C. in the form of an off-white solid.

Medium preparation: 0.0562 g Ciba CGI-784 was dissolved in 0.1 g4-bromostyrene and 0.4 g MTVN under gentle heating. The solution wasmixed with 2.4 g PPGDGE, 1.508 g PETMP, and 0.2 g TDMAMP. Samples wereprepared and holograms recorded as in Examples 2 and 3. Δn values ashigh as 6.2×10⁻³ were attained for thicknesses of 200 μm.

EXAMPLE 6

0.26 g CGI-784 photoinitiator was dissolved in 2.225 g of4-bromostyrene. This solution was mixed with 19 g PPGDGE, 12.2 g PETMP,and 0.34 g 1,8-diazobicyclo[5.4.0]undec-7-ene (DBU). The mixture gelledin 7 minutes, and matrix polymerization was completed after 15 minutes.Multiplexed holograms were successfully recorded in this medium.

EXAMPLE 7

A sample having a material thickness of 940 μm (not including the glassslides) was prepared as follows. 0.75 g Ciba CGI-784 was dissolved in1.50 g MTVN under gentle heating. The solution was mixed with 9.04 gPPGDGE, 5.64 g PETMP, and 0.56 g TDMAMP. Media up to 1 mm thick wereprepared using vacuum holders, as mentioned previously and hologramswere recorded as in Example 2. A Δn of 7.3×10⁻³ was measured for thesample, demonstrating that it is possible to increase the samplethickness while substantially maintaining Δn.

EXAMPLE 8

Five media were prepared to compare effects of differing photoactivemonomers. The media had material thicknesses of 250 μm, and wereprepared as follows:

1) Styrene photoactive monomer: prepared as in Example 2.

2) Bromostyrene photoactive monomer: prepared as in Example 3.

3) Bromostyrene and MTVN photoactive monomers: prepared as in Example 5.

4) MTVN photoactive monomer: prepared as in Example 7.

5) 1-(3-naphth-1-ylthio)propylthio)-4-vinylnaphthalene (NTPVN)photoactive monomer: 0.02 g CIBA CGI-784 was dissolved in 1.2007 g ofPPGDGE. The solution was mixed with 0.4080 g NTPVN, 0.7524 g PETMP and0.1358 g TDMAMP. Samples were prepared as in Example 2.

Thirty five plane wave holograms were angle-multiplexed into the samplesusing the above holographic apparatus. The samples were flood exposedafter writing to react any remaining photoactive species. The refractiveindex contrasts were calculated, and are shown in FIG. 2 (with abest-fit line), using the reference numbers immediately above. FIG. 2shows that from medium 1 to medium 5, an increase in refractive indexcontrast from about 1.6×10⁻³ to about 9×10⁻³ was realized, whilemaintaining a relatively constant level of dimensional stability. (adecrease of ˜0.3% in the thickness of the medium). The increasedrefractive index contrast exhibited by the medium containing1-(3-naphth-1 -ylthio)propylthio)-4-vinylnaphthalene is to be expected,given the presence on the monomer of 2 index-contrasting moieties.

(The NTPVN was prepared as follows:

Preparation of 1-(3-(naphth-1-ylthio)propylthio)naphthalene. A solutionof 20.7 g (0.1 mol) of 1-bromonaphthalene in 200 mL of ether was cooledto −78° with stirring, and 40 mL of BuLi was added. The temperature wasallowed to rise to −20° C., and lowered back to −78° C., at which time3.2 g (0.1 mol) sulfur was added. The temperature was allowed to rise to10° C., and lowered back to −78° C., at which time 14.8 g (0.05 mol)1,3-diiodopropane was added. As the mixture was warmed to roomtemperature, a sluggish reaction was noted by thin layer chromatography.The mixture was heated for 4 hours at reflux in the presence of 50 mL ofTHF, and worked up (after cooling) with aqueous NaOH. The organic layerwas dried with MgSO₄, filtered, concentrated, and chromatographed on 100g of silica gel eluting with 2 L of 0-30% CH₂Cl₂ in hexane. A 900 mLproduct band gave 5.5 g of a white solid, indicated to be pure by NMR.

Preparation of 4-(3-(naphth-1-ylthio)propylthio)-1-naphthaldehyde. 3.9 gof the above product and 1.42 g dimethylformamide were mixed with icecooling, followed by the addition of 2.8 g of P₂O₃Cl₄. The mixture washeated at 100° C. for 2 hours, cooled to ambient temperature, andhydrolyzed by adding 2.5 g of NaOH in 50 mL of ice water, heating to 40°C., and stirring the mixture at ambient temperature. When the organicmaterial became dispersed, it was extracted into ether, dried, filtered,concentrated, and chromatographed with 750 mL of a hexane-CH₂Cl₂gradient, followed by 10% EtOAc in CH₂Cl₂. A total of 2.0 g of startingmaterial, 1.3 g of yellow oily product, and 0.2 g of dialdehyde wereobtained. The product yield was 31%, or 63% based on the consumedstarting material.

Preparation of 1-(3-(naphth-1-ylthio)propylthio)-4-vinylnaphthalene. 1.3g of the above product was added to a Wittig reagent (prepared from 1.4g of methyltriphenylphosphonium bromide and 3.3 mmol (1 equiv) of BuLiin 30 mL of THF at 0° C. to room temperature over 1 hours and recooledto 0° C.). After stirring overnight at ambient temperature, 1.6 mL ofMeOH was added, and the solution was concentrated and extracted withligroin as for the MTVN. The extract was partially concentrated to about10 mL, diluted with CH₂Cl₂ to homogeneity, and chromatographed on 20 gof silica gel, eluting with 1:1 hexane:CH₂Cl₂. The yield was 1.1 g (84%)of a viscous yellow oil, indicated to be pure by NMR. After drying 30min. at vacuum, the material was immediately blended into the mixturefor medium preparation.)

Other embodiments of the invention will be apparent to those skilled inthe art from consideration of the specification and practice of theinvention disclosed herein.

What is claimed is:
 1. An optical article comprising: athree-dimensional cross-linked polymer matrix, and one or more polymersformed from one or more photoactive monomers, wherein the matrix wasformed by a polymerization reaction selected from cationic epoxypolymerization, cationic vinyl ether polymerization, cationic alkenylether polymerization, cationic allene ether polymerization, cationicketene acetal polymerization, epoxy-amine step polymerization,epoxy-mercaptan step polymerization, unsaturated ester-amine steppolymerization, unsaturated ester-mercaptan step polymerization,vinyl-silicon hydride step polymerization, isocyanate-hydroxyl steppolymerization, and isocyanate-amine step polymerization, wherein atleast one of the photoactive monomers comprised a moiety, other than themonomer functional group, that is substantially absent from the polymermatrix, wherein the polymer matrix and the one or more polymers formedfrom the one or more photoactive monomers are compatible, wherein thematrix has a thickness greater than 200 μm, and wherein the polymermatrix was formed, in the presence of the one or more photoactive.monomers, by a polymerization reaction independent from a reaction bywhich the one or more photoactive monomers were polymerized.
 2. Anoptical article comprising: a three-dimensional cross-linked polymermatrix, and one or more polymers formed from one or more photoactivemonomers, wherein the matrix was formed by a polymerization reactionselected from cationic epoxy polymerization, cationic vinyl etherpolymerization, cationic alkenyl ether polymerization, cationic alleneether polymerization, cationic ketene acetal polymerization, epoxy-aminestep polymerization, epoxy-mercaptan step polymerization, unsaturatedester-amine step polymerization, unsaturated ester-mercaptan steppolymerization, vinyl-silicon hydride step polymerization,isocyanate-hydroxyl step polymerization, and isocyanate-amine steppolymerization, wherein at least one of the photoactive monomerscomprised a moiety, other than the monomer functional group, that issubstantially absent from the polymer matrix, wherein the polymer matrixand the one or more polymers formed from the one or more photoactivemonomers are compatible, wherein the matrix has a thickness greater than200 μm, wherein the polymer matrix was formed, in the presence of theone or more photoactive monomers, by a polymerization reactionindependent from a reaction by which the one or more photoactivemonomers were polymerized, and wherein the one or more photoactivemonomers are selected from acrylates, methacrylates, acrylamides,methacrylamides, styrene, substituted styrenes, vinyl naphthalene,substituted vinyl naphthalenes, vinyl ether mixed with maleate, thiolmixed with olefin, vinyl ethers, alkenyl ethers, allene ethers, keteneacetals, and epoxies.